PicoCTF - MatchTheRegex

Description: How about trying to match a regular expression?

1. The homepage consists of an input box where we can try putting our regular expression, aka regex, to try and match the flag:

   

2. When trying test as a test to see how this works we get a wrong match! Try again! message:

   

3. We could try a site like regex101 and build a regex that match the general picoCTF flag structure, such as the following:

   

4. Unfortunately, that did not work. We can take a look at the page’s source code:

   

5. As it seems it only needs a string sequence that matches the pattern in the comment, i.e., ^p.....F!?:

   1. We need a sequence starting with the letter p (^p).
   2. Followed by any 5 characters (.....).
   3. Followed by the letter F (F).
   4. And we can have an exclamation mark (or not) at the end (!?). The ? signifies that the preceded character is optional.

   So any string sequence that matches the above pattern will give us the flag: